1. Technical Field
This application relates to tire pressure diagnostic systems for vehicles. In particular, the application relates to a tire pressure diagnostic system for vehicles for determining a change in the inflation pressure of a vehicle tire using wheel rotation signals in combination with a plurality of vehicle position inputs.
2. Related Art
Low tire detection systems that monitor tire pressure and profile height of the tire are known. The apparatus required for such systems generally includes a sensor and a complicated wiring harness for each tire. For trucks and other large vehicles, which may include up to 18 or more tires, the wire harness structure is even more complicated and raises the cost of the vehicle.
Systems are also known that use effective rolling radius calculations to determine when the radius of a tire changes. For example, a tire that is flat or has low pressure has an incrementally smaller effective rolling radius than an adequately inflated tire. In such systems, wheel displacement sensors may be used to measure the angular displacement of each wheel by means of a rotation detector. For example, if a vehicle is traveling at velocity v, the vehicle tire will rotate with an angular speed Ω=v:r, with r being the radius of the tire. A tire having a lower than normal inflation pressure will have a smaller radius and will thus rotate at a higher angular velocity. Thus, the radius of the tire may be determined from the velocity of the vehicle and the number of rotations of the tire.
In one system, position signals from a global positioning system (GPS) receiver are used to calculate a distance of the vehicle. From the distant, a desired distance related parameter of a tire having a predetermined inflation pressure may be calculated. The system may then compare the distance related parameter sensed from each tire with the desired distance related parameter to determine whether each tire is properly inflated. If low inflation pressure is detected, a warning system notifies the operator of the vehicle of low pressure.
Normally, a GPS receiver provides a data set of the co-ordinates of the receiver in space every second. However, the exact path between two received data sets of the moving vehicle may not be known and the second distance may be inaccurate, resulting in a false low tire inflation pressure warning. Moreover, output may be unreliable due to GPS reception problems in tunnels, forests, inner cities, as well as sudden multipath situations that can fake 200 m offsets of the actual position, thereby causing false alarms. GPS needs a start up time until the first valid reception of some minutes. Thus, it is an object of the invention to provide a cost effective, accurate system of detecting low tire inflation pressure.